Improved validity of cellular and molecular biomarkers from whole induced sputum in asthma and COPD

Background: Biomarkers in induced sputum samples are increasingly important in phenotyping and monitoring of patients with asthma and COPD. Whole induced sputum samples, however, are contaminated with saliva as reflected by the presence of squamous epithelial cells. We aimed to improve the validity of sputum biomarkers in asthma and COPD by taking contamination into account. Methods: Total and differential cell counts, levels of soluble markers of inflammation (myeloperoxidase, interleukin-8 and eosinophil cationic protein) and airway permeability (alpha-2-macroglobulin and albumin) in 482 sputum samples, i.e. 247 samples of 29 COPD patients (mean age: 64 yr; FEV1 predicted: 61%) and 235 samples of 25 asthma patients (25 yr; FEV1:102%) were related to squamous cells. Results: For sputum samples with ≤90% squamous cells, absolute cell numbers (TCC) and levels of soluble parameters showed inverse log-linear relationships with percentage squamous cells, indicative of dilution, in COPD and asthma. TCC and soluble parameters were calculated to be reduced 8and 1.4 to 3.2-, and 2.5and 1.3 to 2.1-fold in samples with 48% and 20% (i.e. median %) squamous cells in asthma and COPD, respectively. These log-linear relationships enabled correction of sputum data for saliva contamination, reducing variability of sputum parameters, and improving repeatability of sputum parameters (improved intraclass correlation coefficients) and the discriminative power in two former patient studies. Conclusions: These findings show that adjustment for percentage squamous cells significantly improved validity of biomarkers in whole induced sputum in asthma and COPD, advancing the implementation of cellular and molecular monitoring in airway disease. C H A P T E R 4